Methods for producing porous bodies include metal plating and powder slurry impregnation coating. The metal plating is disclosed in Japanese Patent Publication (KOKOKU) No. 47-10524, and is a method in which a synthetic resin foam having a three-dimensional network structure as exemplified by urethane foam is subjected to conductive treatment with carbon or the like, followed by electrodeposition of a metal layer of nickel or the like in a plating bath and further followed by firing to decompose and burn off the resin, to obtain an electrodeposited-metal porous body to which the pattern of foamed resin has been transferred.
The powder slurry impregnation coating is disclosed in Japanese Patent Publication (KOKOKU) No. 61-53417, and is a method in which a synthetic resin foam having a three-dimensional network structure as exemplified by urethane foam is immersed in a slurry prepared by mixing a metal powder, a thickening polymer and a solvent, to apply the metal powder to the framework of the foam, followed by heating to decompose and burn off the resin and sinter the metal powder, to obtain a metal porous body to which the pattern of foamed resin has been transferred.
According to the powder slurry impregnation coating, as disclosed in Japanese Patent Application Laid-open (KOKAI) No. 62-269724, a porous body can also be obtained on a non-conductive material such as ceramics by a similar method.
The metal plating is currently used as a practical method, although it requires complicated steps when the metal is plated on insulating materials such as foamed resins. For example, it requires conducting treatment or seeder treatment as pretreatment. Also, this method can not obtain a porous body formed of non-conductive substances such as ceramics and a porous body formed of alloys of two or more metals, and has the problem that materials for porous bodies are limited to some metals feasible for plating, such as nickel and copper.
The slurry coating is commonly a method in which a powder is suspended in a solution of a polymeric organic material to form a slurry and a base material is impregnated with it, and has the problem that the powder may settle unless the slurry is well stirred, and on the other hand bubbles may be held therein if excessively stirred, making it cumbersome to manage slurries. This method has an advantage that materials having no conductivity as exemplified by ceramics can be formed into the product so long as they are powdery, without limitation to metals. However, because of surface tension of the slurry, the powder tends to gather at joints in the framework of the base material rather than the framework itself, and it is difficult for the powder to be uniformly soaked in and applied to the base material. This phenomenon greatly affects the strength of the resulting porous body. That is, when the powder is applied in a certain quantity, the framework may be thinly formed and have a low strength. Accordingly, in order to make the framework thick, a precaution is taken in which the slurry for impregnation coating is used in a large quantity to ensure the strength.
As an additional problem, the above method has a disadvantage that thin filmy portions (closed portions) that close up the network structure of the base material tend to be produced. This is a phenomenon that may occur because the slurry tend to form a film, and depends on the viscosity of the slurry. Even when polymers in the slurry are removed so that the base material is impregnated with a mixture of powder and solvent, powder particles may agglomerate in the course of drying to cause such closed portions.